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Page 14 of 23 Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04

Figure 5. Phase change material for thermal protection. (A) Heat flow direction of wearable device with the thermal protective
substrate (TPS); (B) heat-absorbing microspheres composed of PCM. Reproduced with permission [129] . Copyright 2022, Springer
Nature; (C) a wearable device composed of a soft heater and a thermal protective substrate; (D) heat shielding test at pig skin attribute
TPS. Reproduced with permission [130] . Copyright 2019, Wiley-VCH GmbH; (E) advanced thermal skin (ATS) imparted with silver
flake/PDMS serpentine structure (SPS) and sodium-acetate-based hydrogel matrix (SAHM); (F) on-demand solid-to-liquid phase shift
of SAHM. Reproduced with permission [131] . Copyright 2022, Elsevier; (G) fabrication process of Paraffin@Cu microcapsules; (H) optical
and SEM images of Paraffin@Cu/silicone elastomer composite. Reproduced with permission [132] . Copyright 2021, Springer Nature.

of human skin, resulting in improved comfort during strenuous activity [Figure 5E]. Furthermore, unlike
the spontaneous release of latent heat below the melting point during the phase transition of PCMs, the
exothermic process of SPS can only be activated when needed. This selective phase transition of SPS
protects the human body from unwanted temperature discomfort, whereas the heat reactions of
conventional PCMs are uncontrolled [Figure 5F]. Flexibility was achieved with a low modulus variation of
2
3
4.8, a high thermal diffusivity of 0.307 mm /s, and a large latent heat of 94.29 J/cm , surpassing that of
paraffin.

Sun et al. designed highly extensible composite PCMs with an embedded paraffin@copper microcapsule
(PA@Cu) . The nano-Cu metal shell used to encase PCMs through the Pickering emulsion method is
[132]
illustrated in Figure 5G. Because of the alignment of nano-Cu particles on the PCM particle surface, the
nano-Cu shell is elastic. By manufacturing PA@Cu microcapsules with a high PCM encapsulation ratio of
98%, a composite of flexible PCMs was formed. The silicone elastomer (SE) that the PA@Cu microcapsules
were tightly enclosed in demonstrated their great compatibility. The composite PCMs that arise are flexible
and thermally stable. Tensile tests have shown that flexible composites with a considerably greater loading
of 40-wt% PA@Cu have an extensibility of more than 730% [Figure 5H]. Despite the improvement in the
thermal conductivity of SE because of the PA@Cu, the thermal conductivity of the PA@Cu/SE composite is
only 0.212 W/(mK). The improved ability to regulate thermal energy makes these composites suitable for
wearable electronics because they provide superior thermal protection for human skin.